N-alkyl pyrrolidones, particularly N-methylpyrrolidone, are technically very important solvents in industrial petrochemistry and are also used as starting substances for various syntheses.
N-methylpyrrolidone is generally produced industrially using the Reppe process (Chem. Ing. Technik (22) 17, 1950, pp. 361 ff.), in which monomethylamine and gamma-butyrolactone are maintained with a suitable diluent for a certain period of time at temperatures exceeding 250.degree. C. and reacted with each other via a dehydrating catalyst. Due to the freely occurring dimerization of N-methylpyrrolidone as a result of the reaction, the yield is only 85-90%, however.
In U.S. Pat. No. 4,885,371, a process for producing N-methylpyrrolidone is described in which monomethylamine and gamma-butyrolactone are reacted with each other via a borohydride catalyst.
A number of other publications (JP 01-190667, JP 01-186864, JP 01-186863) also describe the synthesis of N-alkyl pyrrolidones, starting from gamma-butyrolactone, with primary, secondary, or tertiary amines. The mean reaction time is 3 h at 250.degree. C.
According to the Soviet application no. 1558903, N-methylpyrrolidone is produced by reaction of gammabutyrolactone and an excess of monomethylamine at 250.degree. C. via a Y zeolite as a catalyst. According to K. Hatada et al. (Bull. Chem. Soc. Japan .50(10), 1977, pp. 2517-2521), a copper-exchanged Y zeolite is used as a catalyst for the same reaction.
In the process of JP 49-020585, N-methylpyrrolidone is obtained by reaction of monomethylamine, diluted with a large amount of water, and gamma-butyrolactone at 250.degree. C. and 2 hours reaction time.
The process of JP 51-042107 uses gammabutyrolactone, methylamine, and water in the ratio of 1:1.4:4 to produce N-methylpyrrolidone as the end product at a temperature of 250.degree. C. and a pressure of 45-50 kg/cm.sup.2. The excess water serves as a carrier for the reuse of unreacted methylamine.
In accordance with JP 49-000259, N-methylpyrrolidone is obtained by heating a mixture of gamma-butyrolactone, alkylamine, and hydrogen for 3 hours at 270.degree. C. A mixture of copper and a metallic oxide (Cu Plus SiO.sub.2, Al.sub.2 O.sub.3, SiO.sub.2 -Al.sub.2 O.sub.3, TiO.sub.2, ZrO.sub.2, or Cr.sub.2 O.sub.3) is used as a catalyst. The reaction takes place in the gaseous phase, and the yield is only about 60%.
According to JP 47-021420, N-methylpyrrolidone is obtained with a yield of 99% when monomethylamine and gamma-butyrolactone are allowed to react with each other in the aqueous phase for a sufficient length of time.
JP 49-020582 describes a process in which monomethylamine is first allowed to react with gammabutyrolactone, followed by a cyclization, such as via an aluminum oxide catalyst.
Finally, DE 2200600 describes a process for producing N-methylpyrrolidone from maleic anhydride, hydrogen, and an amine compound, in which a palladium-carbon catalyst is used. A yield of only at most 78.4% is obtained at a reaction temperature of 275.degree. C. and a pressure of 119 arm. The reaction occurs in an excess of water.
Furthermore, very long reaction times are prescribed and the reaction occurs under very high pressure, so that expensive process-related measures are needed.
All the aforementioned processes have the disadvantage either that the yield is low or that the product is unsatisfactorily impure and must undergo expensive purification following the process. In a manner similar to that described in DE-A 2200600, processes are also disclosed in U.S. Pat. No. 3,109,005, U.S. Pat. No. 3,448,118, and U.S. Pat. No. 3,198,808 in which pyrrolidone or pyrrolidones are produced using a so-called one-vessel process. These processes have the disadvantages previously mentioned. However, the synthesis usually starts from gamma-butyrolactone, which must be previously produced from 1,4-butanediol, maleic anhydride, or other educts, and isolated.
The object of the invention, therefore, is to provide a process in which N-alkyl pyrrolidones or pyrrolidone itself can be produced directly from the corresponding dicarboxylic acids or their anhydrides, without the otherwise common isolation of an intermediate product, and in which a high product yield is achieved while at the same time fulfilling the high purity requirements imposed on the product.